The present invention relates to a turbine having an inner case, an outer case, an annular cavity defined between said inner case and said outer case, and a plurality of guide vanes which are arranged in said annular cavity, each guide vane (1) comprising an inner platform (2), an outer platform (3) and an airfoil (4) extending between the inner platform (2) and the outer platform (3), wherein at least one groove (5) for receiving a sealing means (6) is formed at a side of the inner platform (2) and/or of the outer platform (3), said groove (5) having a predetermined groove width (a), wherein at least one sealing means (6) is provided, which is inserted in the grooves (5) of the guide vanes (1), opposing side walls (7, 8) of said at least one groove (5) each being formed with at least one curved projection (9, 10), wherein the projections (9, 10) define a gap (11) within said groove (5), whose gap width (b) is smaller than said predetermined groove width (a). The at least one sealing means (6) is formed in one piece with the inner case or with the outer case.